This invention relates generally to radio frequency array antennas and more particularly to radio frequency array antennas adapted to form a plurality of simultaneously existing beams of radio frequency energy.
It is known in the art that a radio frequency array antenna may be arranged to produce a plurality of simultaneously existing beams of radio frequency energy. If such an array antenna is properly designed, each one of the beams has the gain and bandwidth of the entire antenna aperture. According to the art, a desired number of simultaneous beams may be obtained by connecting each antenna element in the array through a different constrained electrical path to a plurality of feed ports, the constrained electrical path being made up of an electromagnetic lens which equalizes the time delay of the electromagnetic energy between any given one of a number of feed ports and all points on corresponding planar wavefronts of either transmitted or received energy. One such antenna is described in U.S. Pat. No. 3,761,936, "Multi-Beam Array Antenna," inventors D. H. Archer et al, issued Sept. 25, 1973 and assigned to the same assignee as the present invention.
While such array antenna has been found quite satisfactory in many applications, it is sometimes necessary that such array antenna have sidelobes lower than those obtainable with a single electromagnetic lens. While such sidelobes are theoretically achievable by tapering the field amplitude across the array aperture, such levels are seldom achieved due to deviations in the aperture field amplitude and phase from the theoretically designed values. Such deviations are generally attributable to such things as mutual array element coupling and reflections within the electromagnetic lens. Conceptually, one method which might be used to correct the amplitude and phase deviations is through the insertion of a variable attenuator and phase shifter serially with each one of the array elements, such attenuators and phase shifters being adjusted to achieve the proper aperture distribution. However, the use of such arrangement would provide proper adjustment for only one beam at only a single frequency.